Linear predictive speech coders which operate according to the aforesaid multi-pulse principle are known to the art, from, for instance, U.S. Pat. No. 3,624,302, which describes linear predictive coding (LPC) of speech signals, and also from U.S. Pat. No. 3,740,476 which teaches how predictive parameters and predictive residue signals can be formed in such a speech coder.
When forming an artifical speech signal by means of linear predictive coding, there is generated from the original signal a number of predictive parameters (a.sub.k) which characterize the synthesized speech signal. Thus, there can be formed with the aid of these parameters a speech signal which will not include the redundancy which is normally found in natural speech and the conversion of which is unnecessary when transmitting speech between, for instance, a mobile and a base station included in a mobile radio system. From the standpoint of conserving bandwidth, it is more appropriate to transfer solely predictive parameters instead of the original speech signal, which requires a much wider band-width. The speech signal regenerated in a receiver and constituting a synthetic speech signal can, however, be difficult to comprehend, due to a lack of agreement between the speech pattern of the original signal and the synthetic signal recreated with the aid of the prediction parameters. These deficiencies have been described in detail in U.S. Pat. No. 4,472,832 (SE-A--456618) and can be alleviated to some extent by the introduction of so-called excitation pulses (multi-pulses) when forming the synthetic speech copy. In this case, the original speech input pattern is divided into frame intervals. Within each such interval there is formed a given number of pulses of varying amplitude and phase position (time position), on the one hand in dependence on the prediction parameters a.sub.k, and on the other hand in dependence on the predictive residue d.sub.k between the speech input pattern and the speech copy. Each of the pulses is permitted to influence the speech pattern copy, so that the predictive residue will be as small as possible. The excitation pulses generated have a relatively low bit-rate and can therefore be coded and transmitted in a narrow band, as can also the prediction parameters. This results in an improvement in the quality of the regenerated speech signal.
In the case of the aforesaid known methods, the excitation pulses are generated within each frame interval of the speech input pattern, by weighting the residue signal d.sub.k and by feeding-back and weighting the generated values of the excitation pulses, each in a separate predictive filter. The output signals from the two filters are then correlated. This is followed by maximization of the correlation of a number of signal elements from the correlated signal, therewith forming the parameters (amplitude and phase position) of the excitation pulses. The advantage of this multi-pulse algorithm for generating excitation pulses is that various types of sound can be generated with a small number of pulses (e.g. 8 pulses per frame interval). The pulse searching algorithm is general with respect to the positioning of pulses in the frame. It is possible to recreate non-accentuated sounds (consonants), which normally require randomly positioned pulses, and accentuated sounds (vowels), which require more collected positioning of the pulses.
One drawback with the known pulse positioning method is that the coding effected subsequent to defining the pulse positions is complex with respect to both calculation and storage. Furthermore, the method requires a large number of bits for each pulse position in the frame interval. The bits in the code words obtained from the optimal combinatory pulse-coding algorithms are also prone to bit-error. A bit-error in the code word being transmitted from transmitter to receiver can have a disastrous consequence with regard to pulse positioning when decoding the code word in the receiver.